1. Field of the Invention
The present invention generally relates to programmable logic devices, and more particularly, to a programmable logic device having adjacent logic elements that can be selectively combined to perform higher order logic functions that can not be performed alone by a single logic element.
2. Description of Related Art
A Programmable Logic Device (PLD) is a semiconductor integrated circuit that contains fixed logic circuitry that can be programmed to perform a host of logic functions. In the semiconductor industry, PLDs are becoming increasingly popular for a number of reasons. Due to the advances of chip manufacturing technology, application specific integrated circuits (ASICs) designs have become incredibly complex. This complexity not only adds to design costs, but also the duration of time needed to develop an application specific design. To compound this problem, product life cycles are shrinking rapidly. As a result, it is often not feasible for original equipment manufacturers (OEMs) to design and use ASICs. OEMs are therefore relying more and more on PLDs. The same advances in fabrication technology have also resulted in PLDs with improved density and speed performance. Sophisticated programming software enables complex logic functions to be rapidly developed for PLDs. Furthermore, logic designs generally can also be easily migrated from one generation of PLDs to the next, further reducing product development times. The closing of the price-performance gap with ASICs and reduced product development times makes the use of PLDs compelling for many OEMs.
Most PLDs contain a two-dimensional row and column based architecture to implement custom logic. A series of row and column interconnects, typically of varying length and speed, provide signal and clock interconnects between blocks of logic on the PLD. The blocks of logic, often referred to by such names as Logic Elements (LEs), Adaptive Logic Modules (ALMs), or Complex Logic Blocks (CLBs), usually include one or more look up table (LUTs), programmable registers, adders and other circuitry to implement various logic and arithmetic functions.
Almost all current PLDs are based on a four (4) input LUT architecture. A four input LUT enables the implementation of a four-variable logic function. Logic functions implemented in PLDs are, however, very often of greater complexity and define more than four variables. In such situations, more than one LUT is needed to implement the function. With a function with six variables for example, anywhere from two to five LUTs may be required. The more LUTs required to implement a given logic function, the fewer LUTs are available for performing other logic. Current PLD architectures are therefore relatively inefficient when implementing complex logic functions.
A higher efficiency PLD architecture having adjacent logic elements that can be selectively combined to perform higher order logic functions that can not be performed alone by a single logic element is therefore needed.